Plasma arc systems are widely used for cutting metallic materials and can be automated for automatically cutting a metallic workpiece. In general, a plasma arc system includes a plasma arc torch, an associated power supply, a remote high-frequency (RHF) console, a gas supply, a positioning apparatus, a cutting table, a torch height control, and an associated computerized numeric controller. FIG. 1 shows an example of a plasma arc system.
In operation, a user places a workpiece on the cutting table and mounts the plasma arc torch on the positioning apparatus to provide relative motion between the tip of the torch and the workpiece to direct the plasma arc along a processing path. The user provides a start command to the computerized numeric controller (CNC) to initiate the cutting process. The CNC accurately directs motion of the torch and/or the cutting table to enable the workpiece to be cut to a desired pattern. The CNC is in communication with the positioning apparatus. The positioning apparatus uses signals from the CNC to direct the torch along a desired cutting path. Position information is returned from the positioning apparatus to the CNC to allow the CNC to operate interactively with the positioning apparatus to obtain an accurate cut path.
The power supply provides the electrical current necessary to generate the plasma arc. The power supply has one or more dc power modules to produce a constant current for the torch. Typically, the current can be set to discreet values. The power supply has a microprocessor, which regulates essentially all plasma system functions, including start sequence, CNC interface functions, gas and cut parameters, and shut off sequences. For example, the microprocessor can ramp-up or ramp-down the electrical current. The main on and off switch of the power supply can be controlled locally or remotely by the CNC. The power supply also houses a cooling system for cooling the torch.
The gas console controls flow of plasma and shield gases to the torch. The gas console houses solenoid valves, flow meters, pressure gauges, and switches used for plasma and shield gas flow control. The flow meters are used to set the preflow rates and cut flow rates for the plasma and shield gases. The gas console also has a multi-inlet gas supply area where the required plasma and shield gases can be connected. A toggle switch can be used to select the plasma gases. The plasma and shield gases are monitored by gas pressure gages. In order to operate the gas console, all settings must be manually selected.
The RHF console houses a high frequency starting circuit that is used to fire the torch. The RHF console also houses a cathode manifold used to interface power and coolant leads between the power supply and the torch. The power and coolant leads and a pilot arc lead make up a shielded torch lead set which connects with the torch. In addition, gas lines are also supplied to the torch to supply gas.
The torch height control sets the height of the torch relative to the work piece. The torch height control, typically, has its own control module to control an arc voltage during cutting by adjusting the standoff, (i.e., the distance between the torch and the work piece), to maintain a predetermined arc voltage value. The torch height control has its own external control module to control the standoff. The torch height control has a lifter, which is controlled by the control module through a motor, to slide the torch in a vertical direction relative to the work piece to maintain the desired voltage during cutting.
The plasma arc torch generally includes a torch body, an electrode mounted within the body, passages for cooling fluid and cut and shield gases, a swirl ring to control the fluid flow patterns, a nozzle with a central exit orifice, and electrical connections. A shield can also be provided around the nozzle to protect the nozzle and to provide a shield gas flow to the area proximate the plasma arc. Gases applied to the torch can be non-reactive (e.g. argon or nitrogen) or reactive (e.g. oxygen or air).
In operation, the tip of the torch is positioned proximate the workpiece by the positioning apparatus. A pilot arc is first generated between the electrode (cathode) and the nozzle (anode) by using, for example, a high frequency, high voltage signal from the RHF console. The pilot arc ionizes gas from the gas console passing through the nozzle exit orifice. As the ionized gas reduces the electrical resistance between the electrode and the workpiece, the arc transfers from the nozzle to the workpiece. The torch is operated in this transferred plasma arc mode, which is characterized by the conductive flow of ionized gas from the electrode to the workpiece, to cut the workpiece.
The plasma arc system as described above has high cycle time. First, a torch operator must know some basic cutting parameters, such as the material to be cut, the thickness of the workpiece, and the plasma gas to be used. Then, the operator must review a series of tables found in books to manually set many parameters such as the power settings on the power supply or the gas flow on the gas console. Having to look up additional parameters takes time and may result in operator error as manual input can be inaccurate.
In addition, some components such as the torch height control and the power supply have their own control, which can be redundant. Furthermore, there is no feedback mechanism between the components of the plasma arc system to optimize the operation of the plasma arc system.